Interpretive Summary: The tremendous increases in crop yields during the 1960s and 1970s, called the Green Revolution, were associated with important changes in plant traits. New plant varieties were shorter and had stiffer stems. A consequence of this change was that the ratio of grain weight to total plant weight, which is called the harvest index, was greatly increased. Much research for crop improvement now focuses on the need to select plant types with high harvest indices. This paper explores the concept that a more fundamental change associated with yield increases in the Green Revolution was the ability of plants to accumulate large amounts of nitrogen. From an historical perspective, changes in crop yield and harvest index appear to have also occurred prior to the 20th century, and an analysis of the link between harvest index and nitrogen requirements showed that these changes in harvest index must have been closely linked to changes in the available nitrogen for crop uptake. This paper concludes by arguing that the first condition for increased yields is not increased harvest index, but rather an increased ability of plants to accumulate nitrogen.

Technical Abstract:
Increasing plant harvest index (HI), the ratio of grain weight to total plant weight, has been an important trait associated with the dramatic increases in crop yields that have occurred in the twentieth century. The objective of this review was to examine from an historical perspective changes in crop HI, and to consider the importance of associated changes in crop N accumulation. While it is clear that in modern times HI has increased, there is evidence that plant selections prior to the 20th century also resulted in changes in HI. There have been at least two social or environmental factors that have influenced these changes. One was the value of grain as compared to the straw. Straw production was a high priority under historical farming systems, thereby making low HI a desirable crop trait. A second factor was N availability to support production of high grain yields. Under conditions where N accumulation is low, there is an advantage in having low HI because limited N can be partitioned into the low N tissue of the vegetative components, producing large amounts of plant mass. On the other hand, an essential requirement for increasing grain yield and HI of crops is to have an increase in crop N accumulation.